1. Field of the Invention
This invention relates to a process for making methylphosphonic dichloride by the chlorination of dimethyl methylphosphonate in the presence of selected catalytic agents.
2. Description of Prior Art
Methylphosphonic dichloride (also known as methyl dichlorophosphine oxide, methylphosphonyl dichloride or MPOD) is a well known chemical intermediate. For example, it may be converted into dimethyl pentaerythritol disphosphonate, which is useful as a flame-retardant. See U.S. Pat. No. 4,411,842 which issued to Hechenbleikner et al on Oct. 25, 1983. Further, MPOD may be converted into compounds having insecticidal activity. See U.S. Pat. No. 4,213,922, which issued to Maier on July 22, 1980.
Several synthetic routes for making MPOD have been suggested. See U.S. Pat. No. 4,411,842, noted above, for description of a number of different methods. One well known method is the uncatalyzed reaction of dimethyl methylphosphonate with thionyl chloride under reflux temperatures. See React. Inorgan. Metal Org. Chem., 4, 417 (1974). The authors of this cited article report that MPOD may be obtained in a yield of 98% of theory. But, upon repeating their experiment, the patentee of U.S. Pat. No. 4,213,922 found that this product could only be obtained in yields from about 25 to 50% of theory. This finding is confirmed in the Comparison experiments set forth below. Moreover, this uncatalyzed method has the commercial disadvantage of being very slow. To improve the rate of reaction and yields, it was proposed in U.S. Pat. No. 4,213,922 that certain nitrogen-containing compounds, namely, selected N,N-disubstituted formamides, selected tertiary amines and selected N,N-disubstituted phosphoric acid triamides, be added to the reaction mixture in small catalytic-like amounts. But, these organic compounds or catalysts may be difficult to remove from the product.
Another catalytic method for preparing organophosphonyl dichlorides is discloses in U.S. Pat. No. 3,775,470 which issued to Vogel on Nov. 27, 1973. That patent teaches that organophosphonyl alkyl esters may be reacted with phosphorus pentachloride in the presence of a catalyst selected from a lithium halide or a mixture of a lithium halide and an alkali metal or alkaline earth metal halide. However, the reported yields of this catalyzed reaction are relatively low (i.e. 30-74% yields) and MPOD was not prepared. Thus, the production of highly pure MPOD at a reasonable cost remains a need.